A strain wave gearing is provided with a rigid internally toothed gear, a flexible externally toothed gear, and a wave generator. The externally toothed gear is ellipsoidally flexed by the wave generator, and caused to mesh with the internally toothed gear in, for example, two locations. In this case, the externally toothed gear has 2n (n: positive integer) fewer teeth than the internally toothed gear. When the wave generator is caused to rotate, the position at which the two gears mesh moves in the circumferential direction. For every one rotation of the wave generator, relative rotation is produced at an amount of an angle corresponding to the difference in the number of teeth between the two gears. The strain wave gearing is generic as a gear drive.
The strain wave gearing was invented by C. W. Musser (U.S. Pat. No. 2,906,143), and since that time, a variety of inventions concerning strain wave gearings have been made by many researchers, including Musser and the present inventor. A variety of inventions particularly relating to the tooth profiles of strain wave gearings have also been proposed. Among these, the present inventor has proposed the invention of a tooth profile design method for deriving the addendum tooth profile of two gears in contact over a wide area by using a technique in which tooth meshing of the internally toothed gear and the externally toothed gear is approximated by a rack (JP-A 63-115943). The addendum tooth profile of the externally toothed gear and the internally toothed gear derived by this tooth profile design method is a tooth profile in which “countermovement-type meshing” (countermovement contact) is performed.
There is demand for further advancement in the performance of strain wave gearings. To respond to this demand, the strength and wear resistance of the strain wave gearing must be further enhanced. In particular, the wear resistance of the tooth surface must be improved. It is therefore effective to use, as the tooth profile of the internally toothed gear, a tooth profile in which passing-type meshing (passing contact) is performed. A tooth profile in which passing-type meshing is performed has excellent properties for maintaining a lubricating oil film between the tooth surfaces and is more capable of preventing or suppressing wear of the teeth surfaces due to the breakage of the oil film than a tooth profile in which countermovement meshing is performed.
The inventor of the present invention proposed a strain wave gearing provided with a tooth profile in which passing-type meshing is performed (JP-A 07-293643, WO 2005/124189). In this strain wave gearing, the number of teeth of the externally toothed gear is set to 2n fewer than the number of teeth of the internally toothed gear, the movement locus obtained by approximating, using rack meshing, the meshing of the teeth of the externally toothed gear with the teeth of the internally toothed gear is calculated, and a similarity curve obtained by similarity transformation of a part of the movement locus is used to set the tooth addendum profile of the two gears.
“Passing-type meshing” is defined as meshing such that, when considering a state in which the end points of the linear elements of the two tooth profiles come into contact with the contact points of the tooth profiles, the linear elements of the two tooth profiles are positioned on the same side relative to the contact point. “Countermovement-type meshing” is defined as meshing such that, when considering a state in which the end points of the linear elements of the two tooth profiles come into contact with the contact points of the tooth profiles, the linear elements of the tooth profiles are positioned on opposite sides, with the contact points sandwiched therebetween.